1. Field of the Invention
This invention is concerned with the preparation of single crystals from the congruent melt of dual component metallic fluorides for use as laser emission materials.
2. Description of the Prior Art
Crystals of the formula ARF.sub.4 have been used as laser emission materials prior to this invention. LiYF.sub.4, in particular, has been grown by a process based on the accepted proposition that the material melts incongruently. "Phase Equilibria in the System LiF-YF.sub.3, " by R. E. Thoma et al., J. Phys. Chem. 65, 1096 (1961). This proportion of incongruency is extended to many rare earth compounds, LiF--RF.sub.3, where R ranges from Eu to Ho. "The Rare Earth Halides," by R. E. Thoma, Progress in Science and Technology of the Rare Earths, Vol. 2, edited by LeRoy Eyring (Pergamon, 1966). Accepting the incongruency behavior, LiYF.sub.4 has been grown. "Single Crystal Growth and Some Properties of LiYF.sub.4," by W. A. Shand, J. Cryst. Growth, 5, 143 (1969). However, the incongruent growth process has two outstanding drawbacks. The first is a forced slow rate of growth, necessitated by the composition mismatch, and the second is the change of growth temperature as a function of the amount of crystal formed from a given starting amount of melt. In the case of LiFY.sub.4, incongruent growth is conducted on the Li rich LiYF.sub.These drawbacks are avoided in our congruent growth process which yields crystals of vastly improved quality at a substantially faster growth rate.
According to E. P. Chicklis et al. in Research and Development Technical Report TRECOM-00130F, Sanders Associates, Inc., January, 1973, entitled "Development of Multiply Sensitized Ho:YLF as a Laser Material," crystals of .alpha..beta.--YLF (LiY.sub.1-x-y-z Er.sub.x Tm.sub.y Ho.sub.z F.sub.4) are grown by the Top-Seeded Solution technique, a modification of the Czochralski technique, in an atmosphere of highly purified helium. In the system LiF--YF.sub.3, incongruently melting LiYF.sub.4 crystallizes as the primary phase over the composition range 51-75 mol % LiF. Although in the corresponding system LiF--ErF.sub.3, LiErF.sub.4 is congruently melting, in the mixed crystal system LiYF.sub.4.LiErF.sub.4 the incongruent melting behavior of LiYF.sub.4 prevails. Thulium and holmium are not present in sufficient amounts to effect the observable phase equilibrium. E. P. Chicklis et al "Development of Multiply Sensitized Ho:YLF as a Laser Material", Research and Development Technical Report TRECOM-0013-F Sanders Associates, Inc, January 1973.
In U.S. Pat. No. 3,649,552 it was disclosed, by one of the applicants herein, that an HF purge during crystal growth would lead to the production of ultra high purity rare earth fluoride crystals and solid state solutions of rare earth fluorides and alkaline earth fluorides. The process disclosed has become known as the "Reactive Atmosphere Processing" method. We have found that this process can be utilized to remove the peritectic character of ARF.sub.4 compounds, thereby allowing one to promote crystal growth from the congruent melt and form dual component single crystals of rare earth fluorides and alkali fluorides. This discovery is contra to published literature and commonly accepted beliefs of those who are skilled in the art of rare earth halide crystal growth.
The "Reactive Atmosphere Process" was applied to the growth of calcium fluoride-rare earth fluoride fluorescent compounds in U.S. Pat. No. 3,769,230. The present invention discloses that the techniques utilized to grow Ca.sub.2 Er.sub.5 F.sub. 19 can be adopted to grow congruent crystals of equimolar alkali metal-rare earth fluorides such as LiYF.sub.4 contra to published literature which states that these materials melt incongruently.